My betta is about 1.5 years old, and while I was away at school my sister noticed she started to get a lump on her left side. When I got home the water was not very clean and it did not look as if my betta had pooped recently. I did a water change, and for the next 3 days she didn't poop. BUT she has been eating when you feed her, which i have limited because she seems constipated. I have NOT tried epsom salts but I did feed her peas the past two days which, she pooped once and it was a cloudy orange color. Her scales are starting to raise it looks like on the swollen parts.
For the most part she is active, but recently she has started to lay right under the water's surfaces. I believe it is constipation, but normally how long does it take to go down? and If it is constipation, wouldn't she stop eating?

Constipation may not make a fish stop eating. I know mine wouldn't. I suggest 3 teaspoons per gallon of epsom salt, teamed up with 78-82 degrees water that is cleaned daily, and the salt re-added every water change. Fast her for a few days as well to see if that helps.

Is it just on the left side or is it the whole stomach? If its off to the side it could be some sort of tumor and not a problem with constipation.

Peas are not actually good for Bettas to eat since they actually can't digest them. What the pea does is push what's in it's way out, forcing whatever is in the fish's digestive system out of the body and if it gets stuck by say, an abscess or whathaveyou then it can kill your fish.

A Betta will almost always try and eat. It will be in your fish's best interest if you fast it for about 2 to 3 days then try feeding frozen Daphnia or frozen Brine Shrimp. Both Daphnia and Brine Shrimp are natural laxatives and can help you out if it's constipation that your Betta is suffering from.

Now, if the scales are starting to raise it could be Dropsy, but that's usually all over the body and not just on the bloated part. Can you get more photos of your Betta so we can see?

You mentioned you saw poop, my Bettas poop orange usually but that's because of the color of food they are fed. What did the poop look like? Was it stringy or more round? If it was stringy then your Betta may not have any of these issues and might have internal parasites which could cause the bloating.

Some bettas will stop eating if the constipation gets bad enough. Really, that's on you to regulate because they are poor self regulators.

How much do you feed and does she get a fast day a week?

I agree with Sena's original suggestion. If the bloat doesn't go away after a few days of fasting and epsoms or you actually see raised scales I would also introduce an internal antibiotic. Ideally Kanaplex or Maracyn Plus (not to be confused with Maracyn or Maracyn II). Use a phone book or yellowpages.com and search hard. If you can't find them Furan 2 has some potential to work.

Also just want to add that overfeeding, feeding poor quality wheat heavy pellets, flakes and cool water all can lead to this. Make sure she's getting a fast day a week. Make sure the first three ingredients are fish - like NLS Betta or Omega One Betta Buffet. Don't feed flakes. Don't overfeed, split the feed up into two smaller meals and fast one day a week. Make sure she's heated up. For most bettas. 78-79 is a sweet spot. She should be a minimum of 76 and a maximum of 82